Fuel cells have been proposed as a clean, efficient and environmentally responsible power source for electric vehicles and various other applications. One example of a fuel cell is the Proton Exchange Membrane (PEM) fuel cell. The PEM fuel cell includes a membrane-electrode-assembly (MEA) that generally comprises a thin, solid polymer membrane-electrolyte having an electrode including a catalyst on both faces of the membrane-electrolyte.
The MEA generally comprises porous conductive materials, also known as gas diffusion media (GDM), which distribute reactant gases to the anode and cathode electrode layers. Fuel, such as hydrogen gas, is introduced at the anode where it reacts electrochemically in the presence of the catalyst to produce electrons and protons. The electrons are conducted from the anode to the cathode through an electrical circuit disposed therebetween. Simultaneously, the protons pass through the electrolyte to the cathode where an oxidant, such as oxygen or air, reacts electrochemically in the presence of the electrolyte and catalyst to produce oxygen anions. The oxygen anions react with the protons to form water as a reaction product.
The MEA is generally interposed between a pair of electrically conductive contact elements or bipolar plates to complete a single PEM fuel cell. In typical operation, the MEA is known to expand and contract with changes in humidity and temperature. Compression retention systems are typically designed in a manner effective to offset the strains produced by membrane swelling that can occur with membrane expansion and compressive stress relaxation in the fuel cell stack. In conventional fuel cell stacks, for example, the MEA is known to expand by up to about 50% of its original thickness in operation.
Compression retention systems act to minimize an over-compression of the fuel cell stack and are designed to maintain a desired contact pressure between bipolar plates, gas diffusion media, and catalyst layers. A limited amount of compression set of the GDM occurs under typical operational loads of the fuel cell stack. However, when excessive compression loads are applied to the GDM, the force can physically degrade the GDM by fracturing carbon fibers or breaking up binders that bind the carbon fibers together to an undesirable extent. Therefore, it is generally desirable for an appropriate compression load to be maintained and provide a desired electrical resistance, but not to exceed the desired range during operation of the fuel cell stack.
It is stated in U.S. Pat. No. 5,484,666 to Gibb et al. that conventional compression systems have consisted of tie rods extending through and between endplate assemblies and secured with fastening nuts. Springs threaded on the tie rods and interposed between the fastening nuts and the endplates have been used to apply resilient compressive force to fuel cell stacks in the stacking direction. Fuel cell side plates having controlled tensile compliance are also reported in U.S. Pat. Appl. Pub. No. 2006/0040166 to Budinski et al. It is stated in Budinski et al that the compression forces on a fuel cell stack can be controlled by incorporating at least one spring element into a side plate.
In U.S. Pat. No. 5,789,091, Wozniczka et al. further reports a mechanism for securing a fuel cell stack in an assembled and compressed state which includes at least one compression band that circumscribes end unit assemblies and interposed fuel cells in the fuel cell stack. Murphy et al. in U.S. Pat. No. 6,040,072 also reports an apparatus for securing an electrochemical cell stack that includes a banding member.
Assignee's co-pending U.S. patent application Ser. No. 11/638,283, hereby incorporated herein by reference in its entirety, describes a compression retention system including at least one generally planar strap forming at least one bend, the at least one strip extending from a first end to a second end unit of the fuel cell stack. Also, assignee's co-pending U.S. patent application Ser. No. 11/591,377, hereby incorporated herein by reference in its entirety, describes a compression retention system composed of at least one tie rod extending from an upper end unit to a lower end unit. The tie rod is disposed outside of the fuel cell stack and fastened to the end units by brackets secured to the end units. A spring is interposed between at least one of the fastener and the end units.
There is a continuing need for a compression retention system for electrochemical fuel cells that optimizes a mass, volume and thermal efficiency of the fuel cell system, provides substantially uniform compression during fuel cell stack operation and with a variety of stack sizes, and militates against a degradation of the gas diffusion media in the fuel cell stack.